The invention relates to undulators which serve as a source of electromagnetic radiation called below also light, which is generated from a particle stream (for example of electrons) passing through the undulator and to a method of operating such an undulator. Undulators are used particularly for the generation of x-rays in synchrotron radiation sources.
World with numerous attempts have been made to construct undulators from permanent magnets in such a way that the polarization direction of the light emitted by the undulators can be changed by mechanical movements. An overview of the techniques can be found in H. Onuki and P. Elleaume, Undulators, Wigglers, and their Applications, chapter 6, Polarizing undulators and wigglers, Tayler and Francis, 2003. According to the state of the art described therein two ways are known by which the polarization direction can be changed:                by mechanical displacement of the permanent magnets or,        by division of the undulator and manipulation of the beam between the undulator parts.        
The first solution requires expensive mechanical structures to permit movement of the magnets under the high forces effective on the magnets. The electron synchrotron BESSY in Berlin for example uses permanent magnet undulators with mechanically variable polarization structures. A variant of this equipment is disclosed in JP 103 03 999 A. The second solution has only limited applicability for normal operation, that is, it can be used only in connection with low radiation energies and is therefore without importance in practice.
Immediately after the disclosure of the superconductive planar undulators in R. Rossmanith and H. O. Moser, Study of a Superconductive in-vacuo Undulator for Storage Rings with an Electrical Tunability between K=0 and K=2, Proc. European Accelerator Conference, 2000, Vienna, there were speculations whether it would not be possible to wind superconductive undulators with helical polarization. The first technical comment was provided by R. P. Walker, who was at that time director of Elettra, Triest, New concept for a Planar Superconducting Helical Undulator, 18th Oct. 2000. A further conceptual sketch for a helical undulator was provided by R. Pitthahn and J. Sheppard, SLAC, Use of a Microundulator to Study Positron Production, 5th Feb. 2002.
A further summary is found in a presentation of Shigemi Sasaki, Argonne, Design for a superconducting planar helical undulator, ESRF, June 2003, wherein the author approves the idea to extend the concept of the superconductive planar undulators to helical undulators but states that it is not clear how one could change the polarization direction.
Based on this prior art and knowledge, it is the object of the present invention to provide an undulator and a method for the operation of an undulator which does not have the disadvantages and limitations mentioned above. Particularly a superconductive undulator is to be provided which permits a change and adjustment of the polarization direction of the synchrotron radiation without mechanical movement. The arrangement is to permit for example a switch-over of the polarization direction of the synchrotron radiation from linear to circular or to change the helicity direction, the helicity defining the direction of rotation of the electric field.